Russian President Vladimir Putin, whose government has long denied having any knowledge of the attack, initially claimed that the two men's names "mean nothing to us", then said that they were civilians.
The second of two Russians who Britain says poisoned former Russian spy Sergei Skripal and his daughter has been named as a military doctor for Russian intelligence by a United Kingdom investigative website.
Sergei Skripal and his daughter Yulia Skripal became critically ill after being exposed to a nerve agent in March.
Bellingcat's identification process included multiple open sources, testimony from people familiar with the person, as well as copies of personally identifying documents, including a scanned copy of his passport.
Bellingcat says he was recruited to the GRU intelligence service during his medical studies and that until September 2014, Mishkin's Moscow home address was the same as the GRU headquarters.
Bellingcat said the real name of the man previously known as "Alexander Petrov" was Alexander Yevgenyevich Mishkin.
Britain's Daily Telegraph newspaper also named Mishkin in a separate report published on Monday.
In late September, Bellingcat reported that "Boshirov" is actually Col. Anatoliy Chepiga, a highly decorated officer with the GRU, Russia's intelligence service.
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The father and daughter survived after a lengthy hospital stay in intensive care.
The group said it would provide forensic evidence and other information it used to conclude Petrov was Mishkin on Tuesday.
Investigative news outlet Bellingcat has tracked down what it says are the true identities of the two suspects, who claim they were only tourists.
Mishkin was born in July 1979 in the village of Loyga in the Archangelsk district of northern Russian Federation, and until September 2014 his registered home address in Moscow was the same as the headquarters of the GRU, Bellingcat said.
Alexander Petrov and Ruslan Boshirov told RT's editor-in-chief they had nothing to do with the Skripals' poisoning.
Footage released by London police of the two suspects in the poisoning of Sergei Skripal. They said they went to the city to visit its cathedral.
The second suspect in the attempted poisoning of a former Russian secret agent has been identified by an investigative website.